Recently, a new process for making polyetherester resins from polyethers was described in U.S. Pat. No. 5,319,006. The process reacts a polyether with a cyclic anhydride such as maleic anhydride in the presence of a Lewis acid catalyst. While it is unclear precisely what chemical mechanism is occurring, the net effect of the reaction is to insert the anhydride randomly into carbon-oxygen bonds of the polyether to generate ester groups in the resulting polyetherester resin. The polyetherester resin may then be combined with a vinyl monomer such as styrene and cured to provide a polyetherester thermoset.
Later, it was discovered that strong protic acids (i.e., acids having a pKa less than 0) and metal salts thereof will also catalyze this type of insertion reaction (see U.S. Pat. No. 5,436,313). Dicarboxylic acids may also be substituted in whole or in part for the anhydride (see U.S. Pat. No. 5,436,314).
The ability to prepare thermosettable polyetheresters by random "insertion" of anhydrides and carboxylic acids into polyethers provides a convenient way of making many unique polyetherester intermediates. These polyetheresters often have favorable performance characteristics compared with polyesters made by conventional esterification processes. Unfortunately, the "insertion" process does not work particularly well with high melting aromatic dicarboxylic acids such as isophthalic and terephthalic acids. Such acids have limited solubility or miscibility in the polyether-containing reaction mixture, even at the relatively high reaction temperatures typically employed. Aromatic dicarboxylic acids are commonly formulated into conventional unsaturated polyester resins to impart good mechanical properties and chemical resistance to thermosets made from the resin.
A two step process for making polyetheresters having a high content of aromatic ester recurring units is described in U.S. Pat. No. 5,612,444. In the first step, a low molecular weight polyether polyol is reacted with an aromatic dicarboxylic acid to produce a polyester intermediate. In the second step, the polyester intermediate is reacted with an anhydride or aliphatic dicarboxylic acid in the presence of a catalyst effective to promote random insertion of the anhydride or dicarboxylic acid into polyether segments of the polyester intermediate. While this two step process has proven to be quite useful, particularly for the preparation of polyetherester resins containing relatively high levels of isophthalic acid, it does have certain limitations. In particular, the incorporation of relatively large proportions of recurring units derived from terephthalic acid is still quite difficult due to the much higher melting point of terephthalic acid as compared to other aromatic dicarboxylic acids such as isophthalic acid. Thus, it would be extremely desirable to develop new polyetherester processes which would facilitate the inclusion of terephthalic acid at high levels in order to further enhance the performance of the polyetherester in thermoset formulations.
New ways to make unsaturated polyester resins (UPR) are also needed. In particular, the industry would benefit from efficient ways to incorporate high-melting aromatic dicarboxylic acids such as terephthalic acid into UPR. Although terephthalic acid is relatively inexpensive and offers resins good water resistance, it is seldom used to make UPR because of its high melting point and poor solubility in organic materials. The industry also needs ways to reduce cycle times in making UPR. Typical commercial resins often require 20 to 24-hour cycle times, which severely limits productivity. Finally, ways to make water-resistant polyester thermosets--ones that retain a high proportion of their tensile and flexural properties even after exposure to harsh aqueous media--are needed.